1. Field of the Invention
Liquid development of electric field images.
2. Description of the Prior Art
In the art of development of electrostatic images to which the present invention pertains (electrostatic images are one common embodiment of electric field images), the first step is to form on a carrier an electrostatic image which consists of many segments in various configurations such, for instance, as lines, characters, letters, symbols, etc., or an object or a picture, the segments being juxtaposed to uncharged segments of the carrier. The charged segments can be created in various fashions such, for example, as providing a carrier with a photoconductive layer, imposing a uniform electrostatic charge on the layer, and thereafter selectively discharging the electrostatic charge by exposure to a modulated beam of radiant energy, leaving an electrostatically charged image. Various other methods can be employed to form an electrostatic image such, for example, as providing a carrier which is a dielectric sheet and transferring a preformed electrostatic charge image to this sheet.
The current most popular system for developing an electrostatic image, particularly for so-called "office copier" machines, is that in which a developer is cascaded across the latent image. The developer is dry and includes a powder which coats carrier particles and is adhered thereto by electrostatic forces that usually are generated triboelectrically. The powder is preferentially attracted electrostatically from the carrier particles to the latent image, leaving powder on the latent image but not on uncharged segments.
In the most widely used office copiers the powder includes a thermoplastic material; the latent image is carried by a photoconductor surface other than an uncoated copy sheet such, for instance, as a selenium drum, and the powder, in the configuration of the ultimate image, is transferred from the drum to the uncoated copy sheet to which it is made to adhere, for example, by the application of heat sufficient to fuse the powder to the sheet but insufficient to deleteriously affect the sheet.
It also has been proposed to use as the carrier for the electrostatic image a flexible copy sheet coated with a photoconductive insulating layer and to fuse the powder directly to such a sheet.
It further has been proposed to develop an electrostatic image with a powder cloud, the powder being suspended in a gas which is blown against the latent image and selectively will deposit on the image largely to the exclusion of deposit on the uncharged segments.
Yet another system for developing electrostatic images employs a magnetic brush in which filaments of magnetically cohered carriers coated with toner or of magnetic toner particles are swept across a surface bearing an electrostatic image and toner is deposited on the image.
All the foregoing techniques are subject to various mechanical drawbacks such as intricate cascading equipment, intricate recirculating equipment for the toner in order to recapture the toner which cascades off the photoconductive layer and brings it back to a zone at which the cascading takes place, and intricate fusing equipment. The powder cloud developing method never has reached the point of widespread commercial acceptance. A further problem with dry developers was that the powder frequently adhered, for various reasons, to uncharged portions of the carrier, thus forming distracting deposits in the background where not such deposits should be present.
An alternate type of development which basically distinguishes from the dry development above described also has come into vogue, although not to the extent of dry development. This is the so-called "liquid" development. Liquid developing machines have been of various types. That most frequently used was one in which the electrostatic image was formed on a flexible carrier sheet having a photoconductive coating such, for example, as zinc oxide in a carrier such as polyvinylacetate. The sheet with the latent image thereon had applied to its entire image-carrying surface a liquid developer which most commonly constituted an organic liquid of a high resistivity with dispersed colored particles. The particles were electrophoretically attracted to the segments of the latent image on the sheet and massed at the charged image segments where they remained after the carrier left the liquid. Because the liquid not only contacted the portion of the sheet carrying the latent image but also the surface surrounding the image, i.e. the background, some particles adhered to the background where they appeared in the finished copy as objectionable spots or an overall background coloration, both of which were highly undesirable. The organic liquid evaporated to leave a dry developed image. The particles were thermoplastic and subsequently were fixed to the sheet by sufficient heat to fuse the particles without damaging the sheet. Surface adsorption and/or penetration sometimes were factors in fixing the particles to the sheet.
Other liquid developing machines employed an LTT principle (liquid toner transfer). In such machines a drum with a repeatedly usable photoconductive surface had an electrostatic image created thereon, after which the entire surface of the drum was progressively submerged in liquid developer at a development zone. Suspended insoluble particles in the liquid were electrophoretically attracted to the image segments on the drum as they traveled through the liquid and massed at the image segments where they remained after the drum left the development zone. The entire surface of the drum was wetted by the developer at the development zone. However, the attraction of the particles to the electrostatically charged image segments arranged the particles on the drum in the desired image configuration within the liquid film that covered the surface of the drum leaving the development zone. Subsequently, this wet image was transferred to an uncoated copy sheet. As in the liquid machines previously described, the LTT machines had an unwanted background coloration as well as unwanted small toned areas in the background.
A liquid developer machine is considerably simpler than a dry developer machine, but the liquid developer machine had many drawbacks such as the volatilization of organic liquid, the wetting of copy paper at background areas and the subsequent drying thereof either in the room or in an area exposed to the room, the necessity for ventilating the room in which the machine was located in order to prevent an objectionable concentration of the vapor of the organic liquid, and the tendency of the uncharged segments of the image to attract developer particles which result in a discoloration of the background and a loss of contrast as well as a loss of edge definition of the developed image.
Because in previous liquid developing machines the entire surface of the object carrying the electrostatic image was wetted by the developer and because this led to darkening of the background, it has been the practice to minimize such drawback by employing a low concentration of particles in the liquid developer, relying for contrast upon massing of particles where a visible image was to be created. Suppliers of toner reduced handling costs by selling liquid toner in a highly concentrated form (high weight-to-weight ratio of particles to liquid carrier). The concentrated toner was considerably diluted to the low concentrations desired for use in the field. A typical working toner had approximately 0.05% by weight of the particles.
Other problems with liquid developing equipment were: the creation of a trailing deposit of the particles beyond the edges of the developed image segments in the direction of travel of the carrier through the equipment, loss of fill (an inability to deposit enough particles from the liquid where a charged segment was of substantial dimension, for example, exceeding 1/4" in any direction) and the susceptibility of the image to smudging due, in part, to incomplete setting of the image because of residual retention of the organic liquid.
Another drawback of liquid developing equipment was economic, and constituted the rather high cost of preparing a liquid developer. The particle size had to be extremely small, the dispersion of the particles in the organic liquid had to be substantially uniform, the particles had to be prevented from settling either upon standing in the machine between uses and over weekends, or on the shelf in a store-house or at a distributor, or even on the consumer's premises. The drawback of settling of the particles has been overcome by some manufacturers by the inclusion of dispersion agents, and by other manufacturers by special formulation of the solid contents of the liquid developer which made the insoluble particles part of an amphipathic molecule of which another part was soluble in the organic liquid. None of these improvements substantially reduced the cost of liquid developers.
It has been proposed to reduce the expense of providing a liquid developer by using water as a carrier in place of the organic liquid. This, also, of course, eliminated the problem of accumulation of vapor of the organic liquid in the vicinity of the office copier. The liquids proposed, generally, have been water containing a water-soluble dye. Due to the employment of a dye as the coloring agent, the problem of settling and dispersion was eliminated. However, to date, there have not been proposed any commercially acceptable liquid developing systems employing an aqueous carrier. The principal problem with these systems has been deposition of some of the liquid developer on uncharged portions of the photoconductor, with consequent loss of a clean background.
The lower cost of water-based liquid developers is not necessarily a controlling factor. Various commercial factors may dictate the use of a water-based developer or of an organic-liquid-based developer.
With respect to the machines which are specifically structured to make use of water-based developers, sundry pieces of equipment have been suggested. One was that disclosed in U.S. Pat. No. 3,084,043 in which a liquid developer was provided in the valleys of a surface, e.g. a roller surface, that included lands. The lands rode on the surface of a photoconductor and, under the influence of electrostatic force created by the segments of the electrostatic image, the developer crept up the sides of the valleys to contact such segments and, theoretically, did not creep up the sides of the valleys associated with segments where no charge existed. This system was proposed for both polar and non-polar developer liquids.
In another system using a liquid developer, see for example U.S. Pat. No. 3,886,900, it was proposed to have a developer roller turn in a tank of developing liquid and to provide an electrostatic field in a gap between the developer roller and the latent-image-bearing photoconductive surface. This field raised a swell, like a standing wave, of developer, which filled the gap between the roller and the photoconductive surface, the image being developed essentially by the same principle as that employed to develop a latent image using a liquid developer in which all portions of the photoconductive surface of a sheet carrying a latent image were treated with the developer, i.e. both charged and uncharged portions of the surface.
Another method proposed was that disclosed in patents such as U.S. Pat. No. 3,383,209 wherein valleys of a developer roller were partly filled with a developing liquid while the lands of the roller touched the photoconductive surface bearing the latent image. The theory of this system was that the developing liquid had a surface tension such as to render the liquid normally lyophobic with respect to the latent-image-carrying surface of the photoconductor, with the strength of the electrostatic field created by the segments of the latent image being sufficient to have the developer wet the portions of the surface having latent image segments thereon. U.S. Pat. No. 3,772,012 shows a similar arrangement.
Still another liquid developing system was proposed in U.S. Pat. No. 3,560,204 in which a developer roller turned partially submerged in a tray containing a liquid developer, the surface of the roller that left the developer passing beneath a photoconductor web having an electrostatic image thereon. The latent image consisted of charged segments juxtaposed to uncharged segments. The spacing, between the film of developing liquid pulled out of the tray by the roller as it turned and the uncharged segments of the image-bearing photoconductive surface, was quite small, not exceeding 3.2 mils. A bias charge was applied to the developer roller that sufficed to draw up unsupported columns of liquid developer each of which was in the shape of its associated uncharged segment to form an ink image on the photoconductive surface. These columns were said to rupture as the photoconductive surface and the film of liquid developer diverged when the roller turned away from its zone of closest proximity to the photoconductive surface.
The processes of all the prior commercial liquid developer systems have been found to leave marks on the background areas where no coloration, theoretically, should exist, and apparently it is for that reason that these processes have not found wide commercial favor despite many sophisticated modifications which have been proposed.
It will be appreciated that, in general, the common drawback of the various approaches to developing an electrostatic image with a liquid developer is the inability to maintain a virgin, i.e. untouched, and, therefore, unchanged, background segment where no color is supposed to exist.
Another liquid developer of the type in which the photoconductive surface bearing a latent image is exposed in its entirety or in part to a liquid is illustrated in U.S. Pat. No. 3,068,115 wherein droplets of a developing liquid are formed and drawn through a dielectric liquid in which the developer droplet is immiscible to said photoconductive surface which is submerged in the dielectric liquid, the droplets being attracted to the charged segments of the photoconductive surface. This arrangement, obviously, is unsuitable for commercial purposes since it is bulky, unwieldy, slow, requires different liquids in the same piece of equipment, and requires subsequent drying of the photoconductive surface.
The foregoing summarizes the main systems known to the inventors, it being appreciated that there are a large number of variants of these systems as well as other systems which are in no way relevant to the present invention.